C. R. HENDERSON 



Cornell Universify 



Chapter 22 



Specific and General Combining Ability 



By general combining ability we mean the average merit with respect to 

 some trait or weighted combination of traits of an indefinitely large number 

 of progeny of an individual or line when mated with a random sample from 

 some specified population. The merit of the progeny is measured in some 

 specified set of environmental circumstances. If maternal effects are present, 

 we must specify that the tested individuals are males. If the tested individu- 

 als are females, the merit of the progeny is a function of both general com- 

 bining ability and maternal ability. 



General combining ability has no meaning unless its value is considered in 

 relationship to at least one other individual or line and unless the tester 

 population and the environment are specified. For example, suppose two 

 dairy bulls used concurrently in an artificial breeding ring each have 500 

 tested daughters, and that it can be assumed that the cows to which the two 

 bulls were mated were a random sample of cows from herds using artificial 

 breeding. Suppose that the mean of the butterfat records of the daughters 

 of the first sire is 410 pounds and of the second sire is 400 pounds. Five hun- 

 dred tested daughters are sufficient to reduce the sampling variance of the 

 progeny mean to a negligible amount. Consequently the general combining 

 ability of the first sire is 410 — 400 = 10 pounds better than that of the sec- 

 ond in this particular population and in this set of environmental circum- 

 stances. The general combining abilities of the two sires might differ by more 

 or by less than 10 pounds if they were used in some other region where both 

 the genotypes of the cows to which they were mated as well as the environ- 

 ment could be quite different from those of the test. 



SPECIFIC COMBINING ABILITY 



We shall define specific combining ability as the deviation of the average 

 of an indefinitely large number of progeny of two individuals or lines from 



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